I have a 1996 XJS Convertible that I just recently purchased.

I am experiencing a high idle. It pegs at about 1,300 RPM. I cannot lower it playing with the throttle body mechanism.

I did the manual base idle reset, and without the Idle Speed Actuator the engine idles at 600 RPM. Once I plugged it back in, I hear some weird flapping noise coming from the idle speed controller and it surges back to 1,300.

Is the idle speed controller bad?

 

Maybe.

High idle on a AJ16 is going to be from a dirty throttle body, the TPS, the ISCV, or rarely, the speed sensor modulator or wire that runs from it to a sensor on the differential. Sometimes it just needs to be reset by the Jaguar PDU. Sometimes it needs one of the above items replaced and then to be reset by the PDU.

 

I hooked it up to a laptop that reads from the OBDII port.

The throttle position seems to be pegged to 16% when my foot is off the gas. I took it up to 94% when I put the pedal to the metal.

Could this mean that the TPS is probably fine? The numbers seem to react accordingly while driving.

I did have the throttle body cleaned yesterday....

Is there a way to test for a faulty stepper motor? Do you know what other devices direct the motor to open/close?

Thanks.

 

I reset the base idle manually by unplugging the IASC three times,
and I did get it to idle at just over 800 RPM, however I started to
notice a dip in power everytime I accelerated out of a stop.
Eventually the dip turned into a stall.

I had that problem with my previous car which pointed to a dying
(or filthy) EGR Valve.

The next morning, the car was idling back at 1,200 RPM. Now it
rythmically fluctuates between 1,248 and 1,216. No more dips in
acceleration from a light. Just high idle.

Any ideas?

 

Computer hook up was needed to reset the parameters.

I also had a differential leak and a bad oil pressure switch.

 

That's not bad at all. With the car revving higher than you where traveling I knew the PDU parameter reset was in your future. I thought the speed sensor modulator might be gone, glad it wasn't.